This invention is related to the art of testing the weatherability and expected lifetime of coatings and coated substrates. More particularly, this invention relates to a method for accelerating the delayed failure of coatings, thus shortening the time necessary to test the samples.
Methods of accelerating the delayed failure of coatings are very important for the evaluation of new coatings as well as the modification or improvement of old coatings. The phrase "delayed failure" as used herein refers to the degradation of coatings, i.e. delamination, yellowing, cracking, etc.; which results after a period of time from prolonged exposure to the elements. These "elements" include, but are not limited to, moisture, humidity, heat and ultraviolet light. Previous methods for accelerating the delayed failure of paints, silicone resin coatings, laminates and other coatings have included Florida tests, QUV tests and stress resistance tests. All tests serve to expose the coatings to severe environments so as to accelerate the degradation which occurs under normal use. These tests permit rapid comparisons of new or modified coatings to existing coatings.
The phrase "Florida tests", as used herein, describes an accelerated test which involves placing a coated sample in a field testing site in Florida, where the heat, humidity and ultraviolet light exposure is unusually high. Coatings will fail under these conditions much earlier than in environments where the coatings will be used. The phrase, "QUV tests", as used herein, describes accelerated laboratory tests wherein a coated sample is alternately exposed to high levels of ultraviolet light at high temperatures and high levels of humidity in darkness. The conditions under Florida tests are relatively moderate, with coated substrates often taking months and years to show any signs of failure. The conditions under QUV tests are more severe and some samples, such as silicone resin coated samples, undergo failure after a number of hours or days. However, with the constant improvement of coatings, the time of failure is getting longer and longer.
Anthony and LeGrand disclose a method of evaluating the stress resistance of coated substrates in copending application, Ser. No. 572,482. These tests are instantaneous in that measurements relating to stress resistance at the time of failure can be obtained without waiting for degradation of the coating. Although this method of analysis does provide useful information concerning the coated substrates and does suggest the relative performance of coated substrates when exposed to the elements described above, the utility of these tests is somewhat limited. The actual degradation of the coating is not tested during stress analysis since the coating is not permitted to degrade. Degradation of a coating is a complicated phenomenon which is effected by factors other than the physical forces reflected by stress analysis. In particular, the effects which ultraviolet light and humidity have on the coated substrate are not reflected by stress analysis.
This invention provides a method for accelerating the delayed failure of coatings and coated substrates caused by exposure to various elements, some of which have been described above. This process permits a rapid evaluation of the degradation of coatings and coated substrates. The time necessary for testing the coatings can be reduced by 1-2 orders of magnitude.